to the Antarctic Megtofis. 205 



of hurried or negligent performance : some of the greatiest ques- 

 tions in meteorology depend on its due execution. 



The next correction, and in some respects the most important of 

 all, is that due to the temperature of the mercury in the barometer 

 tube at the time of observation. To obtain this every barometer 

 requires to have attached to it a thermometer, which in the instru- 

 ments furnished to the observatories dips into the mercury in the 

 cistern, and this must be read and registered at each observation of 

 the barometer. In the appendix will be found a table calculated by 

 Professor Schumacher, which gives for every degree of the thermo- 

 meter and every half inch of the barometer, the proper quantity to 

 be added or subtracted for the reduction of the observed height to 

 the standard temperature of 32° Fahr. 



It must, however, be observed, that this table is only calculated 

 for barometers whose scales are engraven upon a rod or plate of 

 brass reaching from the level of the mercury to the vernier. In 

 many barometers the scale is engraved upon a short plate of brass 

 fixed upon the wooden frame of the instrument, and the compound 

 expansion of the two substances can only be guessed at, but must 

 be obviously less than if the whole length had been of brass. As a 

 near approximation for such imperfect instruments, another table has 

 been placed in the appendix, in which the lesser expansion of glass 

 has been substituted for that of brass. No scientific observer, how- 

 ever, would willingly use such an instrument. 



Although all these corrections are necessary for the strict reduction 

 of registered observations, they ought not to be applied to individual 

 observations previously to registry. In the blank forms of register 

 furnished to the observatories, one sheet is devoted to uncorrected 

 observations, and a second to the corrected ; and it is much to be 

 wished that the proper reductions should be made as soon after the 

 observations as possible. 



2. Thermometers. 



Times of observation. — The external standard thermometer should 

 be observed and registered at the same times as the barometer, and 

 all the register thermometers may be read off' at the time of the 9 a.m. 

 observation, and their indices re-adjusted. But as double maxima 

 frequently, and double minima occasionally, occur, in consequence of 

 sudden changes of temperature, both the thermometers should be 

 occasionally inspected with a view to ascertain whether the motion 

 of either the mercury or the spirit has been reversed in an unusual 

 manner ; and such double maxima or minima should be recorded 

 apart as supernumerary, with the dates and leading features of the 

 case. 



Each observatory has been furnished with a standard thermometer, 

 of which duplicates have been deposited at the Royal Society, and 

 which have been carefully compared with an authentic standard. 

 With this standard it is recommended that all other thermome- 

 ters be carefully and frequently compared, and their differences, 



